This invention relates to techniques for investigating subsurface formations surrounding a borehole and, more particularly, to an apparatus and method for determining the volume of water or the conductivity of the water contained in subsurface formations.
Well logging techniques for determining the resistivity or conductivity of subsurface formations have been in existence for many years. The resistivity or conductivity of formations has traditionally been measured at relatively low frequencies, either by direct detection of signal levels as between spaced electrodes or by induction logging. The measured electrical coductivity of subsurface formations is largely a function of the volume of water in the formations and the conductivity of that water. For conventional resistivity logging devices it has been established that the measured formation conductivity is equal to the product of two quantities, one of the quantities being the water-filled porosity of the formations raised to a particular exponent, and the other quantity being the conductivity of the formation water. For most formations, the exponent is generally about 2, so the measured composite formation conductivity varies approximately as the square of the water-filled porosity and varies approximately linearly with the conductivity of the formation water. Using this relationship, along with additional logging or coring information, it is possible to determine the volume or conductivity of the formation water. For example, if water conductivity for a given formation zone is known, one can determine the water-filled porosity of the formations in the zone using the measured formation conductivity and the relationship set forth. Alternatively, if water-filled porosity were a "known", one could determine the formation water conductivity. In both instances, the determination may be directly utilized to draw conclusions or may be combined with or compared to other logging data to obtain useful information about the properties of subsurface formations.
There has been recently developed a logging device which investigates earth formations surrounding a borehole by injecting microwave electromagnetic energy into the formations and then taking measurements which relate to the propagation of the energy in the formations. A form of this device, currently designated as an "electromagnetic propagation" (or "EMP") device is disclosed in the U.S. Pat. No. 3,944,910 of Rau. In operation of the EMP device, injected microwave energy establishes a lateral wave which travels in the formation adjacent the interface between the mudcake and the formations, typically in the so-called invaded zone. Energy from the lateral wave is received at spaced locations and is processed to obtain useful information about the nature of the formations through which the wave energy has passed.
It is an object of the present invention to provide an apparatus and method whereby determinations of conductivity, as obtained from an EMP type of device, can be utilized to determine the porosity of formations and/or the conductivity, apparent conductivity, or saturation of the water in formations surrounding a borehole.